1. Field of the Invention
The present invention relates to a communication system employing an Orthogonal Frequency Division Multiplexing (hereinafter, referred to as ‘OFDM’) technique, and more particularly to an apparatus and method for generating an OFDM symbol in which a guard interval is inserted.
2. Description of the Related Art
In general, data are carried by a predetermined carrier when the data are transmitted by a wireless communication system. Further, in order to transmit data by means of a predetermined carrier, it is necessary to modulate the data. Such modulation may employ various techniques, one representative of which is an OFDM technique.
The OFDM technique is a modulation technique of 4th Generation Mobile Communication Systems, which it is anticipated being adopting as a digital television standard in Europe, Japan, and Australia, and which was initially recommended as a wireless LAN technique in the early 1990's. According to the OFDM technique, data are distributed to a plurality of carriers spaced at regular frequency intervals from each other, each at an exact frequency, thereby preventing any particular decoder receiving the data from referring to other frequencies which are not transmitted to that particular decoder, that is, providing “orthogonality” among different signals.
The modulation schemes for carriers employed in OFDM systems include multiplexing modulation schemes such as QPSK (Quadrature Phase Shift Keying) for voice broadcasting and 64 QAM (Quadrature Amplitude Modulation) for ground wave digital television broadcasting (which has good band use efficiency). OFDM systems use different modulation schemes according to the characteristics described above. However, in OFDM communication systems, data are transmitted by units based on OFDM symbols regardless of the modulation schemes. Each of the OFDM symbols used in the OFDM communication systems includes an effective symbol interval and a guard interval. Herein, the guard interval is a signal interval for reducing multipath effects or ghost phenomena.
In a system employing the OFDM technique, N pieces of parallel data to be transmitted are carried on N sub-carriers having orthogonality to each other, so that the parallel data are multiplexed. Thereafter, the multiplexed data are summed and transmitted. Herein, when the N pieces of parallel data construct an OFDM symbol, the orthogonality among the N sub-carriers in the OFDM symbol prevents interference among the sub-carrier channels. Therefore, in comparison with an pre-existing single carrier transmission technique, the OFDM technique can increase the symbol period by a factor of the number N of the sub-carrier channels while maintaining the same symbol transmission rate.
Further, in the case of increasing the periods of transmitted symbols and inserting guard intervals between the symbols, the guard intervals can prevent multipath delay or interference between symbols which may be caused by delay of the symbols received through the multipaths and can maintain the orthogonality between the sub-carriers. Moreover, interference among channels, which may be caused by decrease of the orthogonality among the sub-carriers, can be reduced.
A reception terminal for receiving the OFDM signal can perform synchronization by means of the guard intervals inserted among the symbols. The OFDM synchronization includes synchronization for acquiring symbol time offset and synchronization for acquiring frequency offset. Beginning points of the OFDM symbols are estimated in the synchronization for acquiring symbol time offset, and degree of frequency offset of OFDM sub-carriers is estimated in the synchronization for acquiring frequency offset.
In synchronization acquisition utilizing a guard interval of an OFDM symbol, there is a correlation between samples in a guard interval and samples copied to generate the guard interval. Therefore, in a symbol, a position where a correlation value between a guard interval and an interval copied for the guard interval becomes maximum can be considered as the beginning of the guard interval. Usually, after synchronization acquisition is completed utilizing a guard interval, frequency offset is estimated using the acquired information. When the synchronization process for acquiring the frequency offset of the received OFDM symbol is not exact, loss of effective data cannot be prevented. This correlation eliminates the necessity of a pilot symbol in channel estimation. Therefore, the OFDM technique as described above can improve bandwidth efficiency and reduce power consumption.
Due to the reasons described above, OFDM wireless communication systems which employ a technique for inserting guard intervals are being developed in fields requiring high speed data transmission systems, such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Digital Terrestrial Television Broadcasting (DTTB), Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). Also, digital wire communication systems employing a Discrete Multi-Tone (DMT) technique, such as Asymmetric Digital Subscriber Line (ADSL) and Very-high bit rate Digital Subscriber Line (VDSL), necessarily require the technique for inserting guard intervals.
FIG. 1 is a block diagram of a typical OFDM communication system, and shows channel environment as well as blocks of a transmitter and a receiver.
Referring to FIG. 1, the OFDM transmitter 110 includes a mapper 111, a serial-to-parallel (S/P) converter 112, an N-point Inverse Fast Fourier Transform (IFFT) device 113, a Parallel-to-Serial (P/S) converter 114, a guard interval inserter 115, and a digital-to-analog (D/A) converter 116.
The data source to be transmitted is inputted to the mapper 111 of the transmitter 110. The data inputted to the mapper 111 are modulated according to a modulation scheme adopted in each system and then inputted to the S/P converter 112. The S/P converter 112 converts the modulated serial data into N pieces of parallel data xl(k) and then inputs the converted parallel data to the IFFT device 113. Then, the IFFT device 113 performs Inverse Fourier Transform for the N pieces of parallel data and then inputs them to the P/S converter 114. The P/S converter 114 converts the Inverse Fourier Transformed data into serial data xl(n) and inputs the converted serial data to the guard interval inserter 115. The guard interval inserter 115 inserts a guard interval in the data from the P/S converter 114, thereby converting the data xl(n) into data {tilde over (x)}l(ñ) which construct an OFDM symbol. The OFDM symbol outputted from the guard interval inserter 115 in this way is converted into an analog signal in the D/A converter 116, which is then transmitted through a predetermined wireless channel.
The wireless channel through which the OFDM symbol generated through the above-described process is transmitted is a multipath channel 120. When the multipath channel 120 is assumed to be a function Hl(k), the outputted OFDM symbols can be considered as values obtained by calculation using the function. Further, when noise generated in the channel environment is assumed to be {tilde over (w)}l(ñ), the signal received by the receiver has a value obtained by adding the generated noise to the value obtained by the function.
The receiver 130 includes an analog-to-digital (A/D) converter 131, a guard interval remover 132, a serial-to-parallel (S/P) converter 133, an N-point Fast Fourier Transform (FFT) device 134, an equalizer 135, a synchronization and channel estimation device 136, a parallel-to-serial (P/S) converter 137, and a demapper 138.
The received signal to which noise is added while passing the multipath channel 120 is inputted to the A/D converter 131. The A/D converter 131 converts the received analog signal into a digital signal of the form {tilde over (y)}l(ñ) and then outputs the converted digital signal. The digital signal outputted from the A/D converter 131 is inputted to the guard interval remover 132. The guard interval remover 132 removes the guard interval from the inputted digital signal {tilde over (y)}l(ñ) and then outputs a signal in a form of yl(n). That is to say, the signal from which the guard interval has been removed by the guard interval remover 132 includes only the effective OFDM data. The effective OFDM data are inputted to the S/P converter 133. The S/P converter 133 parallelizes the inputted signal and then outputs the parallelized data to the FFT device 134. The FFT device 134 performs an N-point Fast Fourier Transform (FFT), thereby outputting Fast Fourier Transformed parallel data yl(k).
The Fast Fourier Transformed parallel data are inputted to, are channel-equalized in, and are then outputted as a signal such as {circumflex over (X)}l(k) from, the equalizer 135. The signal outputted from the equalizer 135 is inputted to the P/S converter 137. The P/S converter 137 converts the inputted parallel signal into a serial signal and then outputs the serial signal. The converted serial signal is inputted to and demodulated in the demapper 138, so that complete data can be extracted from the signal. Further, the synchronization and channel estimation device 136 acquires symbol synchronization and performs channel estimation for establishing some parameters of the equalizer 135.
During the process described above, when the guard interval inserter 115 inserts the guard interval, the guard interval inserter 115 inserts a Cyclic Prefix (CP), which is longer than the channel impulse response, in a guard interval position between adjacent OFDM symbols, thereby eliminating interference between adjacent symbols and interference between channels.
FIG. 2 illustrates a construction of an OFDM symbol in which a guard interval is inserted according to a conventional CP method.
Referring to FIG. 2, in order to maintain the orthogonality between sub-channels, a guard interval including Tc a number of samples is located before effective data including a number T of samples. In this case, the Tc samples of the guard interval 20 are produced by copying Tc samples 21 to the rear portion of the effective data, which corresponds to the location of the guard interval. Therefore, the size in samples of the OFDM symbol is a sum of Tc, which is the number of samples constituting the CP or guard interval, and T, which is the number of samples constituting the effective data. In this method, Inverse Fast Fourier Transformed effective data (T number of samples) are inputted in series only after a delay corresponding to a sample size (Tc samples) sufficient to allow insertion of the guard interval, and then the rear portion of the effective data is copied as a CP to the delayed interval. Therefore, a delay device (buffer) as many as the FFT/IFFT size T, a memory, or a memory address generator is indispensable and an initial delay of Tc is caused in this method.
For synchronization acquisition in the symbol realized as in FIG. 2, beginning synchronization of the symbol is acquired by performing correlation between the CP during the guard interval Tc and the symbol interval copied to generate the CP, that is, correlation between the guard interval 20 and the copied effective interval 21 in FIG. 2. In this case, a reception terminal must know an exact Signal to Noise Ratio (SNR) for the synchronization acquisition. In other words, if the reception terminal cannot calculate a sufficiently exact SNR, exact synchronization acquisition for the symbol cannot be anticipated and thus frequency offset estimation based on exact synchronization acquisition is impossible.